02-13-2012, 04:50 PM
[quote name='nandadevieast' timestamp='1329140361' post='15823']
Thanks Vim for clarity.
To compare both the lenses, but this time on FF cameras:
135 divided by 2 = 67
85 divided by 1.4 = 60
So 135 has thinner DOF.
But this if the distance is same.
If the frame is same then one will have to stand far with 135, and 85 will have thinner DOF...
Am i right?
[/quote]
Yes, that was from Brightcolour's example. If the physical effective lens diameter is larger, and you have the same AoV, the lens with the larger diameter aperture has less DoF.
Do note I used 1.4 crop factor as an example, for ease of use in my example.
With Nikon cameras, the cropfactor is 1.5X, with Canon, 1.6X, with 4/3s, 2X, etc.
Therefore, with a Nikon camera, starting from FF, for a 135 mm lens one would need 135/1.5 mm = 90 mm on APS-C to get the same AoV. From the same point in space, this would give the exact same framing for APS-C and FF, IOW, a FF camera with a 135 mm lens and an APS-C camera with 90 mm lens.
The second effect is that of DoF. Assume the above 135 mm lens has a maxium aperture of F/2.8, then in that case one would need an aperture of F/1.9, say F/2, to get roughly the same DoF from that point in space.
In your example, assuming Nikon, at 85 mm you'd have a shorter effective FL, and therefore also slightly more DoF at equivalent apertures. However, the differences are really minimal at normal shooting distances; 5 mm is not going to make a lof of difference, especially as the nominal FL can be off by up to 5% from the actual one (more with some manufacturers), and similarly nominal aperture can be off by as much as 15% from the real aperture (1/6 of an f-stop).
Kind regards, Wim
Thanks Vim for clarity.
To compare both the lenses, but this time on FF cameras:
135 divided by 2 = 67
85 divided by 1.4 = 60
So 135 has thinner DOF.
But this if the distance is same.
If the frame is same then one will have to stand far with 135, and 85 will have thinner DOF...
Am i right?
[/quote]
Yes, that was from Brightcolour's example. If the physical effective lens diameter is larger, and you have the same AoV, the lens with the larger diameter aperture has less DoF.
Do note I used 1.4 crop factor as an example, for ease of use in my example.
With Nikon cameras, the cropfactor is 1.5X, with Canon, 1.6X, with 4/3s, 2X, etc.
Therefore, with a Nikon camera, starting from FF, for a 135 mm lens one would need 135/1.5 mm = 90 mm on APS-C to get the same AoV. From the same point in space, this would give the exact same framing for APS-C and FF, IOW, a FF camera with a 135 mm lens and an APS-C camera with 90 mm lens.
The second effect is that of DoF. Assume the above 135 mm lens has a maxium aperture of F/2.8, then in that case one would need an aperture of F/1.9, say F/2, to get roughly the same DoF from that point in space.
In your example, assuming Nikon, at 85 mm you'd have a shorter effective FL, and therefore also slightly more DoF at equivalent apertures. However, the differences are really minimal at normal shooting distances; 5 mm is not going to make a lof of difference, especially as the nominal FL can be off by up to 5% from the actual one (more with some manufacturers), and similarly nominal aperture can be off by as much as 15% from the real aperture (1/6 of an f-stop).
Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....